The Labour peer said Jeremy Corbyn will end up backing the single market

Yesterday Lord Adonis, who served in the Cabinet under Gordon Brown, told a magazine: “Leaving the economic institutions of the European Union, which have guided our destiny as a trading nation for half a century, is a very big step and the importance can’t be over-emphasised.

“To my mind, it’s as big a step that we’re taking as a country as decolonisation in the 1950s and 60s and appeasement in the 1930s."

He instantly drew fire from MPs who demanded that he should be sacked as chairman of the National Infrastructure Commission.

Ex-Tory leader Iain Duncan Smith told The Sun: “I find his comments deeply offensive, as I believe most of the British people will too.”

A spokesman for the Prime Minister said today that Mrs May "completely disagrees" with Lord Adonis but will not force him to quit.

She said: "Andrew Adonis is not a member of the Government. It’s for him to explain his remarks but the PM disagrees with him."